Your wall grows mould about three and a half degrees before it looks wet
Condensation, damp and black mould — and why it keeps coming back
Condensation and mould are decided by the temperature of a surface, not the humidity of the room: the same water vapour amounts to a much higher humidity on the coldest surface in it. There are two thresholds, not one: water appears at 100% surface humidity, but mould becomes possible at 80% — about three and a half degrees warmer.
Written by Abdelmounim El Moussaddar · published by TopDigitalKnowledge · 37 pages, 8 figures, 12 sources, every one cited inline · last verified September 2026
What everybody says
Mould means the wall is wet and needs cleaning.
Mould starts about three and a half degrees before any water is visible, and cleaning treats the evidence rather than the cause.
Water appears on a surface at 100% humidity. But the building standard treats 80% surface humidity as the level at which mould becomes possible — and 80% is reached at a warmer temperature. In a room at 20 °C and 55% humidity: water at 10.7 °C, mould at 14.1 °C.
The hygrometer on your shelf reads the air in the middle of the room. Mould grows on the coldest surface in it, where the same water amounts to a far higher humidity. One room has as many humidities as it has surfaces, and only the worst one decides anything.
The saturation vapour pressure equation and the 80% threshold, derived on the page, plus a table of both your temperatures for every normal room condition — 16 to 22 °C, 40% to 70% humidity. Check the claim rather than trust it.
Geometry, not a defect. An external corner loses heat through more outside wall than it has inside surface to recover it from. Same for window reveals, ceiling edges, and the wall behind the wardrobe — where pulling the furniture five centimetres off the wall is the cheapest fix in the subject.
The old single-glazed window was acting as a dehumidifier in plain sight. Warm the glass and the water does not leave — it moves to the reveal, which is porous, hidden and nobody wipes it. A great many people acquired their mould problem at the moment they fixed their condensation problem.
Measure, make less water, take the water out, warm the surface, and only then clean. Plus a fault finder arranged by what you can see, a printable survey sheet, and the record that settles the tenant-versus-landlord argument with temperatures instead of opinions.
Two cheap things: a hygrometer that records its minimum and maximum, and an infrared thermometer. Together they cost less than one call-out, and the book explains what each one reads correctly and what it does not.
No. Two prevalence surveys quoted are British and are labelled as such. The physics is not: a cold surface in a humid room behaves the same way anywhere, and nothing in the method depends on your climate or your construction.
It tells you how to tell them apart from condensation, and then tells you to stop. Rising and penetrating damp are different problems with different signatures, and nothing in this book will fix a defect letting liquid water in.
No, deliberately. It names no products at all. What it gives you instead is the test that tells you whether a dehumidifier can solve your particular problem or whether the fabric is the cause and no machine will.
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Mould is not a cleaning problem, it is a temperature problem — and the temperature that matters is the wall’s, not the room’s. 12 sources, every one cited on the page where it is used.
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TopDigitalKnowledge · written by Abdelmounim El Moussaddar · last verified September 2026
Every number in this guide is traced to a named, published source.
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